Oriented polypropylene films are known for use in packaging. U.S. Pat. No. 4,604,324 to Nahmias et al. discloses multi-layer polypropylene film structures comprising coextruded layers of a base layer of high stereoregularity polypropylene and a skin layer of comparatively low stereoregularity which contains anti-stick additives. In addition, the skin layer can contain up to 10 wt. % of a wax to improve heat seal characteristics or optical properties of the film.
Metallocene-catalyzed propylene polymers are now available, which have low melting temperatures relative to conventional propylene polymers. For this reason, metallocene-catalyzed propylene polymers have been described as useful as the outer layer heat sealable material of multi-layer films in U.S. Pat. No. 5,468,440 (column 6, lines 32 to 41) to McAlpin et al. The McAlpin et al. patent discusses temperatures at which metallocene-catalyzed isotactic polypropylene plaques or cast samples can be stretched in laboratory equipment. Those temperatures, however, are not suitable for continuous orientation processing.
Other multi-layer films in which the advantages of an outer layer of metallocene-catalyzed propylene polymers are described are found in U.S. Pat. Nos. 5,529,843 and 5,462,807. Although the '843 and '807 patents describe multi-layer films in which an outer layer is made from a metallocene-catalyzed propylene polymer, these patents are silent on the use of metallocene-catalyzed propylene polymers in the core layer of a multi-layer film.
U.S. Pat. No. 5,254,394 to Both et al., discloses a polyolefin film for packaging comprising an isotactic polypropylene base layer and a top layer of syndiotactic polypropylene which has high sealed-seam strength and excellent optical properties. The top layer can contain lubricant additives, such as waxes at levels of up to 2 wt. % relative to each other.
Metallocene-catalyzed polypropylene polymers have been developed under the product names NOVALEN M by BASF, FINACENE by Fina, and EXCEED by Exxon, for example. These metallocene-catalyzed isotactic polypropylenes share the beneficial characteristics of having a very narrow composition distribution. Thus, a large portion of the molecules have a level of atactic defects within a compositionally narrow range, for example, rather than a broad range of atactic defect levels averaging out at the desired level.
These same metallocene-catalyzed isotactic polypropylenes, however, also share the characteristic of having a very narrow molecular weight distribution. That is, a large portion of their molecules have a particular chain length. On the other hand, the more traditionally used isotactic polypropylenes have a broad range of chain lengths, averaging out at the desired level. For good operability in the OPP (oriented polypropylene) process, it has been shown that a broad molecular weight distribution has been highly preferred, if not absolutely necessary. The high molecular weight tails of the Ziegler-Natta catalyzed resins tend to distribute the stretching forces more uniformly, preventing stress concentrations, and ultimately providing more uniform orientation and improved operability. Thus, the metallocene-catalyzed isotactic polypropylene polymers have a narrow composition distribution, which is looked upon favorably for film properties, and a narrow molecular weight distribution, which is looked upon negatively for operability in the OPP process.
For this reason, multi-layer films disclosed in the prior art, heretofore, have generally used traditional Ziegler-Natta catalyzed, broad molecular weight distribution polymers in the core, to distribute orientation stresses more uniformly and avoid film breakage during orientation, relegating the metallocene-catalyzed polypropylenes, with their desired properties, to layers on one or both sides of the Ziegler-Natta catalyzed core resin.
At conditions known to be in use within the general OPP industry, as recommended by sources recognized as knowledgeable in the operation of OPP orientation lines, orientation of an OPP line with metallocene-catalyzed isotactic polypropylene as the core resin would not be expected to proceed as well as orientation when using a broad molecular weight range Ziegler-Natta catalyzed resin.
Surprisingly, we have now discovered that films having metallocene-catalyzed isotactic polypropylene resin as the core material can be produced satisfactorily under certain conditions, discussed more fully below.